It has been proposed in the prior art to provide arc welding helmets with shutters using liquid crystal-based materials fitted in the part of the helmet that is folded down onto the face. In particular, WO0122906 describes welding protection devices using standard liquid crystal (a nematic liquid crystal).
Such liquid crystal-based materials are materials wherein the optical properties can be modified, in particular the birefringence by applying an electrical field thereon. By inserting such liquid crystal-based materials into one or a plurality of cells, positioned between crossed polarizers and/or analyzers, shutters controllable with a voltage are obtained.
These helmets are switchable, i.e. the shuttering thereof is activated by an external optical signal (for example by activating the arc).
Such liquid crystal-based optical shutters may observe at least two states, i.e. at least one on-state, whereby they allow light to pass and a shuttered or off-state, wherein they do not allow light to pass or whereby they only allow a small portion thereof to pass.
In order to fulfill the function thereof correctly, the liquid crystal-based material contained therein must have a number of features.
Firstly, this material must have a satisfactory extinction in the visible range in question. For example, for the production of welding helmets, this extinction should be greater than or equal to 30 dB, in the visible band in question, i.e. 450 to 750 nm.
On the other hand, in the on-state(s), it should not induce excessive extinction. Again in the example of the production of welding helmets, this extinction in the on-state(s) must not be greater than 3 dB in the visible band in question.
Furthermore, such a material must, for numerous applications, be suitable to be provided on a relatively large surface. In the example of the production of arc welding helmets, this optical pupil should be at least equal to 1 cm2 in the case of a square pupil, or have a diameter of at least 1.5 cm in the case of a circular pupil.
It should also be noted that such materials should advantageously have rapid shuttering response times (also referred to as switching times) preferably less than one millisecond for operation in stroboscopic mode on the target temperature range.
Such optical shutters should also have a satisfactory shock resistance, as they will be used under relatively difficult conditions and, for this very reason, have an operating range on relatively wide temperature ranges. For example, in the case of the production of welding helmets, the optical shutter should at least operate within a temperature range between −5 and 55° C.
Various optical shuttering devices are found in the prior art. Besides purely mechanical shutters, which enable complete shuttering but are slow, costly and frequently bulky, shutters using the electro-optic or magneto-optic effect are known. The majority using the electro-optic effect make use of liquid crystal-based materials, for example, which are the least costly. The contrast quality is linked with the features of the liquid crystal-based material, the polarizers and the number of cascaded liquid crystal-based material cells.
In this way, optical shutters using nematic liquid crystal-based materials and other shutters using smectic liquid crystal-based materials are known.
Smectic or nematic PDLC (Polymer dispersed liquid crystal)-based shutters are also known. Such PDLCs consist of the association of at least one liquid crystal (nematic or smectic) and polymer(s). The effect used is a diffusion effect (selective attenuation by means of varying degrees of diffusion). Therefore, it is not necessary to have crossed polarizers and/or analyzers in this case.
However, the optical shutters using liquid crystal-based materials known in the prior art have a number of drawbacks reducing their benefits for some applications such as the production of arc welding helmets in particular.
Nematic liquid crystals and nematic liquid crystal-based PDLCs have mediocre relaxation times of the order of a few dozen milliseconds, incompatible with operation in stroboscopic mode, associated with the use of some electric arc modes.
Smectic liquid crystals (ferroelectric FLC or antiferroelectric AFLC) are more rapid than nematics but have specific defects on large pupils and are more fragile. In addition, they can only be used with small thicknesses.
A manner to prevent defect formation consists of using polymer gel stabilization (PSLC for “Polymer Stabilized Liquid Crystal” or PSAFLC for “Polymer Stabilized Anti-ferroelectric Liquid Crystal”) without adversely affecting the rapid response (or switching) times of smectics (PSFLC or PSAFLC).
PSFLCs or PSAFLCs consist, like PDLCs, of the association of liquid crystal and polymer but differ therefrom in that the polymer is not encapsulated therein in droplet form. In PSFLCs (or PSAFLCs), the liquid crystal and polymer form a composite gel wherein the liquid crystal phase is interconnected.
PSFLC (or PSAFLC) structures have different features according to the polymer content of the mixture. For high concentrations (greater than 10% by weight of polymer), the polymer network structure predominantly affects the features of the liquid crystal.
With reference to FIGS. 1A to 1C, diagrams illustrating a conventional method for manufacturing a cell 100 based on a thickness of PSFLC material 101 between a first 102 and a second 103 optically transparent plate (for example glass) is shown. This cell is for example manufactured to produce a shutter.
In a first step (illustrated in FIG. 1A), the mixture 1010 of smectic liquid crystals and monomer (wherein the monomer content is such that a gel can be formed) is heated to a temperature Tnematic above which the mixture changes from the smectic phase C* (wherein it is at ambient temperature) to the nematic phase whereby, as illustrated in FIG. 1A, the monomer molecules 1011 and the liquid crystal molecules 1012 are substantially oriented in the planes of the first and second optically transparent plates 102, 103.
In a second step (illustrated in FIG. 1B) conventional irradiation is performed, using ultraviolet (UV) radiation, on said mixture 1010 so as to polymerize the monomer and thus obtain a thickness of PSFLC 101 in the nematic phase.
In a third step (illustrated in FIG. 1C), the cell 100 is cooled from Tnematic to ambient temperature and the PSFLC 101 thus changes from the nematic phase to the smectic phase whereby it is organized in the form of smectic layers 1013. So as to finalize the shutter, the cell is placed between crossed polarizers and/or analyzers (not shown in FIG. 1A to 1C).
As illustrated in FIGS. 2A to 2D, in the smectic phase (for example after the cell 100 has returned to ambient temperature following said third step), the appearance of an angle between the directional line n (direction of all the liquid crystal molecules) and the normal line A to the smectic layers 1013 (FIG. 2A) of the cell 100 (the layers 1013 adopting, on entering the smectic phase, an arrangement whereby they are parallel with the normal line to the plane of the first 102 and second 103 plates) obtained causes compression of the layers 1013. In a confined geometry, this compression of the smectic layers 1013 is expressed by a double inclination of said smectic layers which form a herringbone (FIGS. 2B-2C) and/or stripe (FIG. 2D) structure.
This structure is modified when an electric field 120 is applied thereto to switch the PSFLC 101. This modification is characterized by a rectification of the smectic layers 1013 in the thickness of the cell 100 accompanied by an inclination in the other direction (FIG. 2D). This inclination is equivalent to undulation of layers in the plane of the first 102 and second 103 plates.
This structural change induces the formation of a diffraction network 130 diffracting in the plane of the first 102 and second 103 plates of the cell 100 accompanied by parasitic diffusion which may be detrimental or redhibitory for numerous applications, particularly wherein strong illuminations of the cell 100 are applied, for example in the case of the production of a shutter for arc welding helmets (or for an intense laser).
Indeed, this diffraction network 130 (diffracting in the plane of the plates of the cell 100) generates, from the primary incident light beam on the first plate 102 of the cell 100 supplied by the intense light source (welding arc or intense laser), a secondary beam from the second plate 103 which, for example, blinds the user of the arc welding helmet.
In this way, in the case of the arc welding helmet, the presence of specific defects of the use of liquid crystals in the smectic phase (C*) and the existence of regular diffracting and diffusing structures (particularly due to alignment layers, polymer chains, defects) may impair the visual comfort of the user.